When laser welding workpieces, a so-called capillary is formed in the processing zone of the workpiece in which the workpiece is melted locally. From the capillary or a melt bath surrounding the capillary there is discharged metal vapor that rises in the form of small particles in a region surrounding the processing zone caused by to the continuously high energy coupling resulting from the laser beam. A portion of the laser radiation produced by the laser processing head is absorbed by the metal vapor particles, whereby they become heated and, owing to the very high temperature thereof, transmit heat radiation. That is to say, a so-called metal vapor plume is produced.
The spatial form or the extent of the metal vapor plume during the processing operation can vary greatly in terms of time and location. Accordingly, the temperature in the surrounding medium changes and consequently the refractive index changes in terms of time and location to an equally great extent so that there is formed a so-called thermal lens, which brings about a redirection of the laser radiation with high temporal dynamics. The temporally and locally non-homogeneous energy coupling which is thereby brought about in the workpiece typically has a negative effect on the quality of the workpiece processing. This interaction causes weld splashes, weld seam fluctuations in the form of irregular upper seam beads, and/or generally impaired seam geometry.
The formation of a metal vapor plume (but also the propagation of weld gases, smoke, etc.) further involves the problem that the rising metal vapor or the particles contained in the rising weld gases can become increasingly deposited on the laser processing head (for example, on the optical focusing unit). This deposition may lead to impairment of the operation of the laser processing head, for example, by a thermally induced focal point displacement. This danger is particularly evident as a result of metal splashes that occur during the laser processing operation and that are thrown in an uncontrolled manner from the melt of the capillary in the direction of the laser processing head (e.g., the optical focusing unit).
To achieve an improved process result (for example, a better seam quality), it is known from DE 20 2004 017 854 U1 to reduce the interaction of the laser radiation with the metal vapor plume. This reduction is achieved by a region of the focused laser beam located below an optical processing unit being kept free of a mixture of welding gases and warm ambient air to the greatest possible extent by at least one gas flow being directed onto the focused laser beam and passing through the laser beam. To this end, the gas flow can be directed in an oblique manner onto the processing zone and/or in an oblique manner onto a region in front of the processing zone on the workpiece. The gas flow or the gas flows is/are produced by one or more gas nozzles, for example, by an annular nozzle which is arranged coaxially with respect to the laser beam.
To protect the laser processing head or the optical focusing unit (and the above-described gas nozzles or the coaxial nozzle) from metal splashes, DE 20 2004 017 854 U1 also describes introducing a comparatively powerful transverse air flow (cross jet) that is oriented transversely relative to the incident laser beam, is arranged close to the optical unit, and detects and redirects any potential occurrences of metal splashes before they reach and potentially damage the laser processing head or the optical focusing unit.
To minimize the thermal lens effect described above, the cross jet has to be moved as close as possible to the workpiece. This results, on the one hand, in the problem of an interference contour. On the other hand, if the gas nozzles(s) which is/are used are arranged comparatively close to the processing zone of the workpiece, it/they can become relatively easily contaminated by the rising metal vapor (or the metal vapor plume) or by metal splashes thrown from the melt. From a specific degree of contamination, an interruption of the laser operation to clean or to replace the gas nozzle(s) is required to ensure their operation and to prevent impairment of the seam quality. In addition, metal vapor can flow around the cross jet, whereby the contamination already described above as a result of metal splashes may again occur above the cross jet. If the cross jet is blown into the melt bath, the seam quality may be influenced in a negative manner.
US 2009/0134132 A1 further discloses a coaxial nozzle through which is discharged a first rapid gas flow during the laser welding operation, which produces a dynamic gas pressure on a capillary formed on the workpiece so that the capillary remains open and the hydrodynamics of the melt bath is stabilized to prevent the ejection of metal splashes in the direction of the laser processing head. A second slow gas flow which surrounds the first gas flow serves to prevent the contact of the melt with the oxygen of the ambient air.